1. Field of the Invention
The present invention is directed toward a compact light condensing illuminator and particularly to a light condensing illuminator including a collimated light source which transmits light to a ruled mirror reflector system for concentration of the light after passage through a lens.
2. Description of the Prior Art
Illuminators for use in such devices as overhead projectors have of necessity been rather large and of substantial vertical height in order to accommodate a large reflector or a lens having disposed at its focal center a high powered light source in order to provide sufficient illumination power. In such projectors not only is the resultant equipment quite large but also its construction adds to its increased weight and diminishes the prospects of it being a suitable portable instrument.
From an operator's standpoint, it is frequently necessary to place subject material for projection upon the projector stage and either refer to it by pointer or annotate the subject material by marking its surface during the course of any discussion or presentation which the operator may be making. Such projectors, for the most part, have significant side glare which is controllable but generally only through the use of costly specially designed lens systems or louvered screens to prevent glare in the direction of the operator. These added devices or elements also increase the size and weight of the unit.
Projectors of this kind are expensive to manufacture and thus to procure because of their mass and of the cost of parts necessary to provide power to meet acceptable projector standards.
In overhead projectors and other instruments which use large condensing illuminators, a pair of high powered Fresnel lenses are generally disposed in a face-to-face relationship. One inherent undesirable feature of double high powered Fresnel lenses is side glare as hereinbefore mentioned. Such units first receive illumination from a high powered standard light source at one of the Fresnel lenses to collect the widely dispersed light and pass it to the second of the Fresnel lenses to condense the light at some point or distance from the lens. In such high power lenses, there is a great loss of light at the extremities of the lens, which accounts for the low efficiency of high power Fresnel lenses at the edges. Further, vignetting is a serious problem encountered in the use of high power Fresnel lenses. The distances of the light paths from the source filament to the lens vary greatly. When comparing, for example, the path of light traversing the axis of the lamp with the path of light directed toward the extremity of the lens the distance difference is at its maximum. The greater light path distance between the source and the lens edge causes the edge of the lens or projection screen to look substantially darker than its center area.
The use of ruled or grooved mirror optical elements to reflect or otherwise act upon light rays is well known in the art. Planar grooved reflectors have been used, for example, as disclosed in U.S. Pat. No. 3,877,802 entitled "Method of Enlarging Images Without Lenses and Display Devices Utilizing the Same" issued Apr. 15, 1975 for inventor M. Greenspan. In the Greenspan disclosure, an object of the invention, to which the disclosure is so directed, provides for a lenseless system of enlarging images along at least one of the orthogonal axes without depending upon the radiation properties of a point source of light.